Dynamic sizes for GitHub-like implemented

[ghost]

:wrench: Implemented dynamic sizes for GitHub-like identicons.

• Added a bunch of tests, all passing.
• Backwards compatible, as previous 5x5 deterministic test still passes.
• Architected now so more renderers can be added.
  • I added an opts list that can mean things to different render implementations.
  • The current renderer knows of simply the size option.
• README says accepts charlists as input, but it didn't. I added a clause that transforms them to strings and then calls render with the string.

Relates to #2